1. Field of the Invention
The present invention relates, in general, to driver circuits, and, more particularly, to systems and methods for compensating for supply voltage variations to a switched voltage-mode VCM (Voice Coil Motor) power drivers in which an important system variable is controlling the current level in the load.
2. Relevant Background
Switched power driver circuits are widely used to generate power suitable for driving loads such as motors. Switched power drivers turn on and off repetitively to supply regulated current in an efficient manner (i.e., with minimal switching loss). Switched power driver circuits are associated with driver circuits that control, for example, the magnitude by means of the duty cycle of the on and off cycles so as to supply a desired amount of power to a load. In a typical application, a power driver circuit is controlled by a VCM system processor, often implemented as a microcontroller IC, that generates commands to the driver circuit. The driver circuit essentially turns on and off in response to the received commands. When on, the driver circuit supplies current to the load, and when off, the driver circuit cuts off current supply to the load.
VCM loads are permanent magnet motors, making their behavior (e.g., speed and direction) related to the magnitude and polarity of the current supplied. For precision motor control applications such as required in disk drive storage systems, unexpected variation in this magnitude can lead to positioning errors and slower performance. For example, the typical application involves control of a voice coil motor (VCM) that controls position of a head assembly with respect to a disk surface in a hard disk drive. Transient variations in the load current translate into position errors. The disk drive system must compensate for the errors by waiting for the transient condition to subside before writing or reading data from the desired location.
In the case of the driver for a VCM in a disk drive, a typical circuit topology is one that is termed a xe2x80x9cCurrent Minor Loopxe2x80x9d (CML). This refers to a feedback circuit that generates a signal that is proportional to or indicative of the current magnitude in the load. The xe2x80x9cminorxe2x80x9d loop refers to localized feedback in the driver circuit in contrast to the xe2x80x9cmajorxe2x80x9d loop that controls the head position. The current in the load is sensed by an external current sense resistor, for example, that is coupled in series with the load. This resistor is typically a high precision power resistor that is relatively expensive.
The voltage across the resistor is brought into a control IC through an additional pin. This sense voltage is amplified by a special, high CMRR (Common Mode Rejection Ratio) amplifier. This amplifier can be expensive in terms of development costs and added die area. The control IC executes commands that specify a voltage value corresponding to a desired current level. The measured current sense voltage is compared to the requested current command value and a corrected command value is applied to the power drivers to obtain a load current that is proportional to command value. This extra IC pin required to port in the resistor voltage is not desirable in highly integrated circuits due to an increase in package cost. Moreover, the extra IC pin displaces other functionality that could be implemented using the pin. The current sense amplifier also adds cost. When the supply voltage (i.e., the voltage supplied to the voltage-mode driver circuit) varies, the action of the feedback loop eventually maintains the proper current in the load. As higher levels of circuit integration are desired and the reduction of external components and pins is desirable due to cost and circuit area constraints, the CML topology is undesirable.
One method of creating a topology that eliminates the need for the external current sense resistor, additional pin, and current sense amplifier, is a voltage-mode driver. Voltage mode power drivers refer to a class of control circuits that regulate the output voltage as opposed to output current. Voltage mode drivers are desirable because they require fewer device I/O pins to monitor and regulate the supplied power. A switched voltage-mode VCM power driver would not sense the current in the load and therefore would have no means to correct for supply variation. This supply variation in a switched voltage mode drive directly affects the average voltage to the load.
While voltage and current in an inductive load are related, a voltage-mode driver regulates current in the inductive load indirectly by regulating the applied voltage. In this case, the average voltage output of the driver is proportional to the command value. When the mode of operation of the driver is linear or continuous, the output average voltage value can be sensed and used to correct for the variation of supply voltage in a feedback method similar to the CML, but without the external current sense resistor and extra pin.
Another alternative is to use a switched mode driver technique. Switched mode drivers are also known as pulse width modulation (PWM), phase shift modulation (PSM) and other names. Switched-mode drivers can be implemented as current-mode (i.e., use the current minor loop) or as voltage-mode drivers. These switched mode drivers have the advantage of reducing the power dissipation to the driver devices and therefore allow smaller devices and packages.
In a VCM driver that is switched mode and voltage mode the resulting output current to the load is directly proportional to the supply voltage. For example, for a fixed input command, the resulting load current when the supply voltage has dropped 10% is also 10% lower than the current would be at nominal VDD levels. It is desirable to correct for this variation in supply voltage level since this affects the gain and bandwidth in the servo loops. This affects the performance (e.g., tracking accuracy and settling time) of the device in which the driver is being used. The command to the power driver is updated at a rate that is matched to the feedback information rate of the system. In a disk drive this is the servo sector rate. This rate is too slow to effectively correct for supply variation between servo sectors. It is preferable that the driver correct for power supply variations at a much faster rate.
Briefly stated, the present invention involves a power driver having an input for receiving an external supply voltage VDD. A voltage-mode driver is coupled to the power supply voltage and generates a drive signal to a load. A system processor generates commands indicating a programmed voltage output desired from the voltage-mode driver. A comparator (or difference calculation method) compares VDD to a reference voltage to generate an error signal. A combination mechanism generates a modified command using the error signal. The modified commands are coupled to the voltage-mode driver, such that the voltage-mode driver generates a voltage output based upon the modified command.
In another aspect, the present invention involves a method of driving a VCM load. A power supply provides a voltage VDD. A voltage-mode driver is coupled to this power supply and generating a drive signal to a load. Commands indicating a programmed voltage output desired from the voltage-mode driver are generated. VDD is compared to a reference voltage to generate an error signal. The commands are modified using the error signal to produce a modified command and supplied to the voltage-mode driver. The voltage output is generated from the voltage-mode driver based upon the modified command.